Optical glass



Patented F eb. 12, 1952 OPTICAL GLASS William H. Armistead, Corning, N.Y., assignor to Corning Glass Works, Corning, N. Y., a corporation ofNew York No Drawing. Application May 13, 1948,

. Serial No. 26,930

Claims. 1

This invention relates to optical glasses and particularly to highrefractive index glasses suitable for use in optical systems of the typeknown as anomalous achromats.

Anomalous achromat optical systems employ a special combination ofglasses in order to correct chromatic aberration and minimize secondaryspectrum. Such glasses differ as widely as possible in refractive index(nD) preferably at least 0.13, but must have dispersive indices or nuvalues (1/) of at least the same order of magnitude and preferably thehigher refractive index glass also has the higher nu value. Thisnecessitates one of the glasses having both a high refractive index anda high nu value. The nu value is an inverse measure of the dispersivepower of the glass, that is, the higher the nu value the lower thedispersive power.

Most optical glasses do not possess a combination of high refractive anddispersive indices. Various glass compositions containing rare earthoxides and other expensive oxides have been proposed in order to meetthese optical requirements. Such prior glasses are not only excessivelyexpensive but have an objectionable tendency to devitrify, that is,crystallize, during melting 01' working. They also tend to be unstablechemically and are subject to excessive weathering.

An object of my invention is to provide optical glasses which have highindices of refraction and dispersion and are suitable for use inanomalous achromat systems.

A further object is to provide such glasses free from the objectionablefeatures of prior glasses.

' The effectiveness of lonthanum oxide in raising the refractive indexof glass is well known. The amount that can be used, especially inborosilicate glasses, is limited by devitrification tendencies whichrequire the use of special melting techniques to avoid this difficulty.

The use of a few percent of BeO to stabilize such glasses has beensuggested. Its use has been limited, and generally avoided because ofits low solubility in the glass.

I have discovered that CaO has an unexpectedly strong solvent effect onBeO and LazOs in borosilicate glasses. This discovery has enabled me toproduce glasses containing essentially LazOs, BeO, CaO', B203, and SiOz,that are chemically stable, resistant to devitrification and high inrefractive index and nu value. Other bivalent oxides such as BaO and SrOadd to the effectiveness of the CaO. Such other bivalent oxides are notsubstitutes for C210 since their solvent 2 effect, even in largeamounts, is relatively minor compared to that of 0:10.

Broadly my invention consists in new glass compositions which contain5-40% SiOz, 5-40% B203, 2-15% BeO, 10-60% 02.0, 5-35% LazOs. the totaldivalent oxides (R0) being 20-75% andthe total divalent oxides plusLazOa being 55-80%. By divalent oxides I mean oxides of the metals ofthe second periodic group and lead. Other oxides, if desired, may alsobe included in these compositions as follows: up to BaO, up to 60% SrO,up to 60% CdO, up to 20% PhD, up to 30% MgO, up to 60% ZnO, up to 20%B1203, up to 20% SbzOs and one or more of the oxides T102, 2102, H02 andThO2 individually amounting to not over 11%. It is to be noted that theoxides T102, ZrOz, HfOz and ThOz constitute subgroup A or the zirconiumsub-group of the fourth periodic group.

Best results are obtained with glass containing 10-25% SiOa, 10-25%B203, 2-12% BeO, 15-30% CaO, 5-35% LazOs. up to 35% 39.0, up to 35% CdOand one or more of the oxides TiOz, ZIOz, HfOz and ThOz individuallyamounting to not over 11%.

The divalent oxides BeO, CaO, BaO, CdO, SrO, PbO, ZnO, and MgO,especially BeO and CaO, are effective in raising the refractive indexwithout appreciably lowering the nu value and a total R0 content of atleast 20% is necessary. Within individual limitations up to may be usedwithout danger of devitrification. At least 2% BeO is necessary forstabilization purposes. The presence of CaO in amounts greater than 10%and preferably from 15-30% permits the use of up to 15% BeOwithoutdevitrification. CaO, BaO, CdO and SrO maybe used individually inamounts up to 60% without devitrification. Over 20 PbO obj ectionablylowers the nu value. MgO and ZnO are less effective in raisingrefractive index but may be used in amounts'up to 30% and 60%respectively without devitrification. V

LazOa appears to enter the glass structure in the same manner as thedivalent oxides and has,

as previously indicated, a similar effect in raising the refractiveindex. At least 5% LazOa is necessary to effectively raise therefractive index and the presence of CaO permits the use of up to 35%without devitrification. The content of divalent oxide plus Lagos mustbe at least 55 to meet optical requirements and the total may be as highas without devitrification.

Oxides of the elements of the zirconium subgroup of the fourth periodicgroup raise the refractive index and'improve the chemical stability 3 ofthe glass. Their solubility is increased by the presence of CaO and theymay be used individually in amounts up to about 11%. HfOz and ThOz areleast desirable due to cost and T102 tends to lower the nu value.

Alumina lowers the nu value and reduces the solubility of the zirconiumgroup oxides and is preferably omitted although it may be used as an aidin preventing crystallization.

group consisting of 2-15% BeO, up to 30% MgO, -60% CaO, up to 60% ZnO,up to 60% SrO, up to 60% CdO, up to 60% BaO and up to PhD, the selecteddivalent metal.oxides including BeO, 02.0, and CdO in theabove-indicated proportions, the total divalent metal oxides beingbetween 20% and 75%, and the total divalent metal oxides plus LazOabeing between 55% and 80%.

Alkali metal oxides lower the chemical dur- 10 4. A transparent opticalglass comprising 10- ability and except for lithia lower the nu value.8102, 10-25% B203, 5-35% LazOs, up to 11% Hence they should be avoidedunless needed to of an oxide selected from the group consisting ofadjust physical properties. TiOz, ZrOz, HfOz and ThOz, 2-12% BeO, 15-30%The oxides of antimony and bismuth have an CaO, up to CdO and up to 35%BaO, the efiect similar to lead oxide and may be used in 15 totaldivalent metal oxides being between 20% amounts up to about 20% 810203or 20% Bi2O3. and 75%, and the total divalent metal oxides plus Thefollowing compositions in percent by weight LazOs being between 55% and80%. as calculated from their batches illustrate but 5. A transparentoptical glass comprising 5- do not limit the invention: SiOa, 5-40%B203, 5-35% LazOa, and at Table,

I II III IV v VI VII VIII IX R0 45 25 25 45 55 47. 5 BO+L81O 65 55 55 6565 57. 5 60 60 11D l. 700 l. 726 1. 687 1 681 1. 713 1. 702 1. 667 1658 1. 654 P- 51. 6 47. 8 53. 8 49. 7 45. 9 54. 1 55. 3

The above compositions are recited solely to illustrate the glasseswithin the broad limits previously set forth and the high indices ofrefraction and dispersion which are characteristic of these glasses.They are alsocharacteristically resistant to chemical attack anddevitrification.

Examples I to V are glasses whose optical properties and othercharacteristics are such as to make them especially valuable inanomalous achromat systems. These glasses are illustrative of thefollowing range of compositions which have properties equivalent totheabove glasses: S102, 15-20%; B203, 15-20%; La203, 20-30%; BeO about 5%,02.0 about 20%, about 5% of an oxide of a metal of the zirconiumsub-group of the fourth periodic group, the total bivalent oxides being25%-4, the total bivalent oxides plus LaOs being 55-6 5%.

Iclaim:

1. A transparent optical glass having the following compositionz, S102,15%; B203, 15%; LazOa, 20%; BeO, 5%; CaO, 20%; CdO, 20% and ZrOz, 5%.

2. A transparent optical glass comprising 5-40% S102, 5-40% B2O3, 535%Laz03, and at least three divalent metal oxides in the indicatedproportions selected from the group consisting of 2-15% BeO, up to 30%MgO, 10-60% CaO, up to 60% ZnO, up to 60%v SrO, up to 60% CdO, up to 60%BaO and up to 20% PbO, the selected divalent metal oxides including BeO,CaO, and CdO in the above-indicated proportions, the total divalentmetal oxides being between 20% and 75%, and the total divalent, metaloxides plus LazOa being between 55% and 80%.

3. A transparent optical glass, comprising 5-40% Si02, 5-40% B203,,5-35% LazOs, up to 11% ZrOz, and at least three divalent metal oxidesin the indicated proportions selected ,from the least four divalentmetal oxides in the indicated proportions selected from the groupconsisting of 2-15% BeO, up to 30% MgO, 10-60% CaO, up to 60% ZnO, up to60% SrO, up to 60% CdO, up to 60% BaO, and up to 20% PbO, the selecteddivalent metal oxides including BeO, CaO, CdO, and Ba0 in theabove-indicated proportions, the total divalent metal oxides beingbetween 20% and 75% and the total divalent metal oxides plus LazOs beingbetween 55% and 6. A transparent optical glass comprising 5- 40% S102,540% B203, 5-35% LazOs, up to 11% ZrOz, and at least four divalent metaloxides in the indicated proportions selected from the group consistingof 2-15% BeO, up to 30% MgO, l0-

, 60% CaO, up to 60% ZnO, up to 60% SrO, up

the selected divalent metal oxides including BeO, a

to 60% CdO, up to 60% 132.0, and up to 20% Phil/j CaO, CdO, and BaO inthe above-indicated pr portions, the total divalent metal oxides beingbetween 20% and 75%, and the total divalent metal oxides plus LazOzbeing between 55% and 80%.

7. A transparent optical glass comprising 10- 25% S102, 10-25% B203,5-35% LazOz, up to 11% of an oxide selected from the group consisting ofT102, ZrOz, HfO2 and T1102, and divalent metal oxides including 2-12%BeO, 1530% CaO, and up to 35% CdO, the total divalent metal oxides beingbetween 20% and 75%, and the total divalent metal oxides plus LazO:being between 55% and 80%.

8. A transparent optical glass comprising 10- 25% S102, l0-25% B203,5-35% LazOa, up to 11% ZrOz, 2-12% BeO, 15-30% CaO, up to 35% CdO and upto 35% BaO, the total divalent metal oxides being between 20% and 75%,and the total divalent metal oxides plus LazOa being between 55% and80%.

9. A transparent optical glass comprising 15- 20% S02, 15-20% B203,2030% LazOa, about ZrOz, about 5% BeO, about 20% CaO and up to 20% CdO,the total divalent metal oxides being between 25% and 45% and the totaldivalent metal oxides plus LazOa being between 55% and 65%.

10. A transparent optical glass comprising S102, 15-20% B203, 20-30%LazOa, about 5% ZrOz, about 5% BeO, about 20% CaO, up to 20% CdO and upto 20% BaO, the total divalent metal oxides being between and and thetotal divalent metal oxides plus LazOa being between and WILLIAM H.ARMISTEAD.

The following references are of record in the REFERENCES CITED file ofthis patent:

UNITED STATES PATENTS Number

2. A TRANSPARENT OPTICAL GLASS COMPRISING 5-40% SIO2, 5-40% B2O3, 5-35%LA2O3, AND AT LEAST THREE DIVALENT METAL OXIDES IN THE INDICATEDPROPORTIONS SELECTED FROM THE GROUP CONSISTING OF 2-15% BEO, UP TO 30%MGO, 10-60% CAO, UP TO 60% ZNO, UP TO 60% SRO, UP TO 60% CDO, UP TO 60%BAO AND UP TO 20% PBO, THE SELECTED DIVALENT METAL OXIDES INCLUDING BEO,CAO, AND CDO IN THE ABOVE-INDICATED PROPORTIONS, THE TOTAL DIVALENTMETAL OXIDES BEING BETWEEN 20% AND 75%, AND THE TOTAL DIVALENT METALOXIDES PLUS LA2O3 BEING BETWEEN 55% AND 80%.